Homework for Chapter 12

Homework for Chapter 12 - Homework for Chapter 12 -Star...

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Homework for Chapter 12 -Star occupies its position on the main sequence as long at it fuses hydrogen into helium in its core. Eventually, the core stars to run out of hydrogen fuel, and the star begins to die. -main-sequence lifetime of a star equals the amount of time the stars fuses hydrogen in its core. -Which type of star has the shortest main-sequence lifetime? O stars- the most massive stars have the shortest lifetimes. -All stars are born with similar compositions: roughly ¾ hydrogen and ¼ helium, with no more than about 2% heavier elements. A star’s main-sequence lifetime therefore depends on: Its mass, which determines how much hydrogen fuel it has in its core. A star twice as massive as the Sun starts with twice as much fuel. Its luminosity tells us how rapidly it is producing energy by fusing hydrogen into helium. A star twice as luminous as the Sun fuses twice as many atoms each second. -Star with high luminosity and a small mass would have a very short lifetime, and a hypothetical star with a low luminosity and a high mass would have a very long lifetime. -Luminosity of a main-sequence star depends directly on the star’s mass. - luminosity of a main sequence star increases rapidly as the twice the Sun’s mass is much more than twice as luminous as the Sun. If a star is twice as massive as the Sun, its main sequence lifetime will be: About 15% that of the Sun Why is the minimum temperature at which helium can fuse higher than that for hydrogen? Helium nuclei have higher charges, so they repel each other more strongly than hydrogen nuclei.
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Homework for Chapter 12 - Homework for Chapter 12 -Star...

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